1. Field of the Invention
The invention relates to a dielectric resonator and a dielectric resonant component, and more particularly to a dielectric resonator and a dielectric resonant component having a structure in which an inner conductor and an outer conductor are formed inside and outside a dielectric block, respectively.
2. Discussion of the Related Art
As a conventional small-sized resonator which has a high Q (i.e., low loss) and is used for a high frequency band (for example, VHF-SHF bands), a coaxial type dielectric resonator is well known. FIGS. 22(a), 22(b) and 23(a), 23(b) show the configuration of two types of conventional dielectric resonators, respectively, which are disclosed in Japanese Patent Examined Publication (Kokoku) No. HEI 3-70403. FIGS. 22(a) and 23(a) are longitudinal sectional views of the respective dielectric resonators, and FIGS. 22(b) and 23(b) are front views of the respective dielectric resonators.
First, the configuration of the dielectric resonator of FIGS. 22(a) and 22(b) will be described. In FIGS. 22(a) and 22(b), an inner conductor formation hole 5 extending in the axial direction is formed in the center portion of a column-like dielectric block 1. On the entire inner surface of the inner conductor formation hole 5, an inner conductor 2 is formed, and, on the outer surface of the dielectric block 1 other than one end surface 1a, an outer conductor 4 is formed. As a result, the one end surface 1a of the dielectric block 1 functions as an open end surface at which the inner conductor 2 and the outer conductor 4 are not formed. The other end surface 1b of the dielectric block 1 functions as a short-circuit end surface at which the inner conductor 2 and the outer conductor 4 are short-circuited. In the dielectric resonator of FIGS. 22(a) and 22(b), a stepped portion 51 is formed in the inside of the inner conductor formation hole 5. According to the configuration, two resonant portions r1 and r2 having different line impedances are formed with the stepped portion 51 as a boundary. The line length L.sub.1 of the resonant portion r1 which is closer to the open end surface 1a, and the line length L.sub.2 of the resonant portion r2 which is closer to the short-circuit end face 1b are equal to each other. In the dielectric resonator of FIGS. 22(a) and 22(b), the stepped portion 51 is formed so that a spurious resonant frequency does not exist at an integral multiple of the basic resonant frequency.
The dielectric resonator of FIGS. 23(a) and 23(b) has substantially the same structure as that of the dielectric resonator of FIGS. 22(a) and 22(b). Corresponding elements are designated by like reference numerals. In the dielectric resonator of FIGS. 23(a) and 23(b), a stepped portion 101 is formed on the outer surface of the dielectric block 1. The stepped portion 101 is formed for the same purpose as that for the above-mentioned stepped portion 51. The line length L.sub.1 of the resonator portion r1 which is closer to the open end surface 1a, and the line length L.sub.2 Of the resonator portion r2 which is closer to the short-circuit end surface 1b are equal to each other.
In various filters (a bandpass filter (BPF), a band-elimination filter (BEF), etc.), oscillators and the like which are used in an apparatus such as a radio communication apparatus for a high frequency band, a dielectric resonant component which is constituted by a plurality of dielectric resonators is adopted in many cases, thereby improving the characteristics and miniaturizing the apparatus. Conventionally, in order to attain desired frequency characteristics, such a dielectric resonant component is constructed in the following manner: A plurality of separate dielectric resonators such as shown in FIGS. 22(a) and 22(b) or 23(a) and 23(b) are prepared, and arranged within one case. Then, the dielectric resonators are coupled to each other via external coupling elements. However, in such a structure, the number of parts is increased, and the weight is also increased. Therefore, there has been a need for a dielectric resonant component which has a further reduced size and a reduced weight. Especially for a modern mobile communication device (a portable telephone, an automobile telephone, etc.), owing to the conditions of use thereof, it is desirable that a resonator component used therein be small and light.
In response, a dielectric resonant component which has a reduced size and a reduced weight has already been realized by combining a plurality of dielectric resonators into one unit within a single dielectric block. FIG. 24 is an exploded perspective view showing such a conventional dielectric resonant component. In FIG. 24, a dielectric block 1 having a substantially hexahedral shape is provided with, for example, three inner conductor formation holes H1 to H3, and coupling holes h1 and h2 which are disposed between the respective inner conductor formation holes H1 to H3. On the inner surfaces of the inner conductor formation holes H1 to H3, inner conductors are formed, respectively. On the outer surface of the dielectric block 1 other than an open end surface 1a, an outer conductor 4 is formed. So-called resin pins P1 and P3 include resin portions P1a and P3a, and signal input/output terminals P1b and P3b, respectively. By inserting the two resin pins P1 and P3 into the inner conductor formation holes H1 and H3, respectively, from the open end face 1a side of the dielectric block 1, the signal input/output terminals P1b and P3 b are capacitively coupled with the inner conductors of the inner conductor formation holes H1 and H3. A cover 100 is provided for holding the dielectric block 1 and the resin pins P1 and P3, and for covering the open end surface 1a of the dielectric block 1 in order to prevent leakage of the electromagnetic field. After inserting the resin pins P1 and P3 into the dielectric block 1 and placing the cover 100 thereon, all the parts are combined into one unit by soldering the cover 100 to the outer conductor 4. Protrusions 100a and 100b of the cover 100 function as ground terminals when the above-mentioned dielectric resonant component is mounted on a circuit board.
In the dielectric resonators of FIGS. 22(a), 22(b) and 23(a), 23(b), since the outer conductor 4 is not formed on the open end surface 1a of the dielectric block 1, there arises a problem in that the electromagnetic field leaks from the open end surface 1a. Specifically, when a conductor of another circuit element comes close to the open end surface 1a, the circuit element may be adversely affected. The dielectric resonator may be coupled with an external electromagnetic field, resulting in that the desired characteristics of the dielectric resonator cannot be obtained. In addition, when the resonant frequency of the dielectric resonators of FIGS. 22(a), 22(b) or 23(a), 23(b) is to be changed, it is necessary to change the axial length (L.sub.1 +L.sub.2) of the dielectric block 1. Accordingly, for each of various required resonant frequencies, a dielectric block having a different size must be prepared, and thus the parts cannot be standardized. As a result, there also arise problems in that mass productivity is reduced, and that production cost is increased.
By contrast, in the dielectric resonant component of FIG. 24, the electromagnetic field leakage from the open end surface 1a is reduced by placing the cover 100 on the dielectric block 1. However, the provision of the cover 100 is not sufficient to completely prevent the electromagnetic field leakage from the open end surface 1a. This produces a problem in that the electromagnetic field still leaks from the open end surface 1a to interfere with other circuit elements, and also problems in that the number of parts is increased, and that it is difficult to reduce the height of the dielectric resonant component because of the thickness of the cover 100. Furthermore, in the dielectric resonant component of FIG. 24, as in the dielectric resonators of FIGS. 22(a), 22(b) and 23(a), 23(b), when the resonant frequency of each dielectric resonator is to be changed, the size of the dielectric block 1 must be changed. Therefore, there are problems in that the parts cannot be standardized, and that production cost is increased. Moreover, in the dielectric resonant component of FIG. 24, when the degree of coupling or the coupling relationship between the respective dielectric resonators (either the inductive coupling or the capacitive coupling) is to be changed without changing the size of the dielectric block 1, a coupling element such as a capacitor, a coil or the like must be externally connected, thereby producing a problem in that the number of parts is increased.